1. Field of the Invention
The present invention relates generally to image sensors.
2. Description of the Related Art
A variety of modern electronic systems have created a strong demand for low-cost, high-quality, mega-pixel image sensors. In response to this demand, currently-available image sensors generally provide arrays of photodetectors (e.g., photogates or photodiodes) that are coupled to readout structures (e.g., charge-coupled-device (CCD) registers or complementary metal-oxide-semiconductor (CMOS) arrays). The photodetectors generate sense signals in response to incident electromagnetic radiation and the readout structures transport these sense signals to an image-sensor output port.
Currently-available image sensors are typically fabricated in accordance with very large-scale integration (VLSI) techniques. Accordingly, they take advantage of photolithographic processes which can define a large number of semiconductor devices on an underlying semiconductor substrate and also define a multiple-layer interconnect structure over the semiconductor devices to facilitate routing of signals and power among these devices.
Current image-sensor arrangements, however, create a significant conflict because different functional systems of the image sensor demand access through the same internal spaces of the VLSI structure. This conflict is exemplified in FIG. 1 which illustrates a conventional image sensor 20 that carries photolithographically-generated semiconductor devices 22 on a (typically silicon) substrate 24. Arranged over the semiconductor devices is a photolithographically-generated interconnect structure 26 that comprises alternating layers of interconnect lines and insulators.
The interconnect structure 26 connects a portion of the semiconductor devices 22 to form an array of photodetectors 28 which each receive focused electromagnetic radiation 30 from a respective one of an array of microlenses 32 that are positioned in the upper part of the image sensor. Although microlenses are not required in all image sensors, they are often used for increasing the percentage of electromagnetic radiation that falls upon the photodetector array.
The sensed signals generated by each of the photodetectors represents a respective pixel in a detected image and these sensed signals are routed to a sensor output port by a transmission system that is formed with the interconnect structure 26 and other portions of the semiconductor devices 22.
From FIG. 1, it is apparent that the focused incident radiation 30 must pass through the interconnect structure 26 to impinge upon the photodetectors 28. Due to various restraints (e.g., intrinsic properties of silicon and application-imposed boundary constraints), each pixel portion of radiation has asymptotically approached a current lateral dimension 34 on the order of 3 micrometers. In contrast, an exemplary 90 nanometer VLSI fabrication process can form something on the order of 100 semiconductor devices in the same area of the substrate 24.
Accordingly, a substantial number of semiconductor devices and a significant portion of the interconnect lines are excluded from the regions of incident radiation 30. This exclusion imposes a number of penalties (e.g., increased chip size, increased design and fabrication costs and decreased performance features) on the current image sensor art. Although concepts have been proposed (e.g., optically-transparent interconnect lines) to alleviate some of these penalties, they generally add other disadvantages (e.g., increased sensor cost).
The present invention is directed to image sensors that reduce conflicts between different sensor functions. Sensor embodiments of the invention include:
a) a substrate,
b) semiconductor devices and an interconnect structure carried over the substrate,
c) reference and sense electrodes and a photoconductive medium that are carried over the interconnect structure, and
d) vias that couple the reference and sense electrodes to the interconnect structure and semiconductor devices.
The semiconductor devices are thereby connectable by the interconnect structure to realize the functions of a signal conversion and transmission circuit without regard to the sense functions of the photoconductive medium and its associated reference and sense electrodes. Different sensor elements do not demand access through the same internal VLSI spaces and may thus be independently arranged to enhance their respective sensor functions. Coupling is realized by vias whose lateral extent is negligible and which may be laterally moved so long as they contact their intended target.
In addition, sensors of the invention are suitable for a wide variety of sensor applications because they will respond to radiation rays that are orthogonal to the sensor""s upper face and also to radiation rays that are tilted from this relationship, and radiation that would otherwise be attenuated or scattered by passage through associated structures.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.